This is another example where the CMOS analog switch discussed in section 15.1 is used almost exclusively. It works by transferring charge into and out of a capacitor when switches are opened and closed. However, the switching action does produce small transients at the chopping frequency, that can mix with the input signal frequency and produce intermodulation distortion.ġ5.2.3 Noise Considerations for Chopper Stabilized OP AMPSĪ switched capacitor is an electronic circuit element used in discrete time signal processing systems. The bandwidth of A 1 thus determines the overall signal bandwidth, and the input signal is not limited to less than one-half the chopping frequency as in the case of the traditional chopper amplifier architecture. Note in this architecture that the input signal is always connected to the output, through A 1. In integrated IC chopper-stabilized op amps, both amplifiers and the storage capacitors C 1 and C 2 are on a single chip. Similarly, C 2 holds the correction voltage for A 2 during the sample mode. During the auto-zero mode, the correction voltage for A 1 is momentarily held by C 1. This is achieved in the auto-zero mode (switches in “Z” position) by momentarily disconnecting A 2 from A 1, shorting its inputs together, and coupling its output to its own Null pin. Note, however, that A 2 also has an input offset voltage, so it must correct its own error before attempting to null A 1's offset. In the sample mode (switches in “S” position), the nulling amplifier, A 2, monitors the input offset voltage of A 1 and drives its output to zero by applying a suitable correcting voltage at A 1's Null pin. Both amplifiers are considered identical and both have an additional Null input terminal. In this circuit, A 1 is the main amplifier, and A 2 is the nulling amplifier. The output “hold” capacitor, C 4, and the load, R L, must be chosen such that there is minimal V OUT droop during the auto-zero cycle.įigure 15.2.2: Auto-Zero (Chopper-Stabilized) Op Amp It is also assumed that after a steady state condition is reached, there is only a minimal amount of charge transferred during the switching cycles. The R 1-C 1 combination serves as an antialiasing filter. The frequency used to chop is usually between a few hundred Hz and several kHz, and it should be noted that because this is a sampling system, the input frequency must be much less than one-half the chopping frequency in order to prevent errors due to aliasing. When the switches are in the “S” (sample) position, V IN is connected to V OUT through the path comprised of R 1, R 2, C 2, the amplifier, C 3, and R 3. When the switches are in the “Z” (auto-zero) position, capacitors C 2 and C 3 are charged to the amplifier input and output offset voltage, respectively. This is a common example where the CMOS analog switch we just discussed in section 15.1 can be put to good use.
A basic chopper amplifier circuit is shown in figure 15.2.1 below.
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